1. Field of the Invention
The present invention relates to a computer numerical control (CNC) lathe, and particularly to a CNC lathe using a screw rod having a positive thread section and a counter thread section in association with two nuts respectively secured on a tool holder assembly and a worktable assembly to simultaneously shift the tool holder assembly and the worktable assembly in opposite directions along a spindle axis, so as to significantly increase the relatively shifting speed between the tool holder assembly and the worktable assembly, to minimize non-cutting time of the CNC lathe.
2. Description of Related Art
Generally, in order to accomplish the purpose for cutting a workpiece by a tool, a computer numerical control (CNC) tool machine is equipped with facility for feeding the tool toward the workpiece during a cutting operation period and with facility for changing and adjusting the relative positions between the tool and the workpiece during a non-cutting operation period. In the cutting operation, the speed for feeding the tool toward the workpiece is determined by the characteristics of the material to be machined, and thus shall not be changed or increased arbitrarily. However, if the speed for shifting or adjusting the relative positions between the tool and the workpiece during the non-cutting operation period is increased, the overall machining speed and efficiency of the tool machine will be greatly improved.
FIG. 1 illustrates a conventional horizontal lathe having a spindle S horizontally and rotatably disposed on a stationary base secured on a frame of the lathe, for gripping a workpiece to be machined. Since a workpiece to be machined is gripped on one end of the spindle S pivoted on a stationary base, it is rotatable but not shiftable. A tool holder B is applied for holding a tool for cutting the workpiece, and is shiftable relatively to the workpiece. Generally, the tool holder B is reciprocatable along the spindle axis (e.g., Z-axis) by virtue of a screw rod R rotated by a servomotor M in association with a nut N secured on the tool holder B, so that the tool can get closer to or to depart away from the workpiece when the servomotor M rotates the screw rod R clockwise or counterclockwise. The tool holder B is also provided along an axis perpendicular to the spindle axis with a further screw rod, similar to the one pivotally disposed along the spindle axis, so that the tool is shiftable along another direction (e.g., X-axis) relative to the workpiece, when the further screw rod is rotated.
FIG. 2 illustrates a conventional CNC vertical lathe, in which a workpiece to be machined is gripped on a spindle pivotally disposed on a stationary worktable T. Similar to the conventional horizontal lathe as described above, a tool secured on a tool holder B of the conventional vertical lathe is reciprocatable respectively along the spindle axis (Z-axis) and the X-axis respectively by a screw rod/nut mechanism, so that the tool can get closer to or depart away from the workpiece in two-dimensional directions.
FIG. 3 illustrates a conventional CNC reverse spindle vertical lathe having a construction substantially identical with the conventional vertical lathe, excepting that a tool holder B thereof is stationarily mounted on the frame, and a workpiece to be machined is gripped on a spindle pivotally disposed on a worktable T shiftable relatively to the tool. Similar to the conventional vertical lathe as described above, the conventional reverse spindle vertical lathe is reciprocatable along the spindle axis (Z-axis) and the X-axis perpendicular to the spindle axis, respectively by a screw rod/nut mechanism, so that the workpiece is shiftable relatively to the tool in two dimensions.
Referring to the conventional manner of applying a screw rod/nut mechanism, e.g., the transmission mechanism including the screw pair of the screw rod R and the nut N illustrated in FIG. 1, to change the relative positions between a tool holder and a worktable, if one intends to increase the speed for the tool holder to get closer to or to depart away from the worktable, he can only accomplish this purpose by increasing the rotation of the screw rod by using a higher speed servo motor. It is, however, found that none of the existing servomotors available in marketplace would satisfy this need for providing a satisfactory shifting speed between the tool holder and worktable during the non-cutting operation period.
Especially, when the conventional screw rod/nut mechanism is applied to reciprocate the tool holder along a vertical axis, e.g. when the tool holder B is reciprocated along vertical Z-axis by the screw rod R in association with the nut N of the conventional CNC vertical lathe illustrated in FIG. 2, the load of the servomotor M to rotate the screw rod R for upwardly shifting the tool holder B is significantly greater than the load of the servomotor M for downwardly shifting the tool holder B, due to the weight of the tool holder B. This not only reduces the service life of the servomotor, but also is disadvantageous to the control of the operational speed. To overcome such problem as to the servomotor bearing different loads when driving a screw rod to upwardly and downwardly shift the tool holder B, the conventional art applies a chain, wire or the like to draw a balance weight, to balance the weight of the tool holder B, and to avoid the problem that the servomotor has different loads when upward and downward shifting the tool holder B. Though this is feasible and practicable, it makes a lathe complicated in construction, bulky and cumbersome, and disadvantageous for manufacture and maintenance.
Referring to the conventional CNC vertical lathe illustrated in FIG. 2, the tool holder B is shifted upward or downward when the servomotor M clockwise or counterclockwise rotating the screw rod R. Such an operation manner and construction of applying a servomotor M in association with a screw rod/nut mechanism to upward and downward shift a tool holder B, when electric power is unexpectedly off or the servo motor is not energized, the weight of the tool holder B creates a torque to rotate the freely rotatable screw rod R, and is gradually moved downward by the weight of the tool holder B. This would subsequently result in that the tool holder B collides with and damages the workpiece, worktable, or the other parts of the machine.
An object of the present invention is to provide a CNC lathe, in which the relatively shifting speed along the spindle between the tool and the workpiece is the double of the relatively shifting speed of the conventional one of applying a screw rod/nut mechanism to simply shift either the tool or the workpiece. The present invention applies a screw rod having two sections of opposite threads, namely a positive thread section and a counter thread section, in association with two nuts separately secured on a tool holder assembly and a worktable assembly to accomplish the relative movement or shifting of the tool and the workpiece along the spindle axis, so that the non-cutting operation time of a CNC lathe is significantly reduced, and the overall machining speed and efficiency are improved.
A further object of the present invention is to provide a CNC vertical lathe, in which a tool holder assembly and a worktable assembly are reciprocatably disposed along a vertical spindle axis by virtue of a screw rod having two sections of opposite threads, namely a positive thread section and a counter thread section, respectively, in association with two nuts respectively secured on the tool holder assembly and the worktable assembly, so that a servomotor, when rotating the screw rod clockwise or counterclockwise to shift the relative position along the vertical spindle axis between the tool holder assembly and the worktable assembly, is of a substantially identical and constant load, without using any additional balance weight.
A still further object of the present invention is to provide a CNC vertical lathe that can avoid the tool holder to collide with the workpiece or the machine, when the electric power is unexpectedly off or the servomotor is not energized.
Additional objects, operational principle, construction, advantages, construction, and features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements.